Aqua-ammonia absorption systems incorporate, as the major components, an absorber, generator, condenser, and an evaporator. Such systems, well known in the art, are disclosed, for example, in U.S. Pat. Nos. 5,548,971, 5,367,884, and 5,490,393 the descriptions of which, including system components, features and operation, are incorporated herein by reference. These aqua-ammonia absorption systems may be designed and operated as heat pumps, refrigeration equipment, chillers, heating appliances, and chiller-heaters.
The generator of an aqua-ammonia absorption system operates as a distillation column and includes, as components, a boiler, a stripping section or stripper, and a rectifying section. The composition feed to the generator comprising an ammonia rich liquor from the system absorber, is introduced to the generator at one or more feed or inlet points. The boiler is designed to produce liquid and vapor counter-flow coincident with heat input. Prime heat is introduced over a finite length or height of the boiler resulting in significant change in ammonia concentration in the area of heat input. In some systems, the boiler is replaced by a reboiler in which heat is input from a prime energy source but with no substantial ammonia separation other than from vapor separating in equilibrium from the liquid near the bottom of the column. Thus, a reboiler simply returns vapor to the generator column.
The stripping section comprises all sections of the generator column which are below the highest (coldest) feed point. In the stripping section, heat is recovered from solution leaving the bottom of the distillation column with the recovered heat returned to the portion of the column above the boiler. The stripping section comprises three portions: a solution-heated desorber (SHD) and either an adiabatic desorber or a generator-absorber heat exchange (GAX) desorber, and the boiler. The SHD is that portion of the stripping section which extracts heat from weak solution, i.e., solution from the bottom of the generator column, before the weak solution is routed to the absorber. The adiabatic desorber of the stripping section has no heat input and is typically located between the coldest feed point and the SHD. The GAX desorber receives heat from the absorber, either by heat transfer using weak liquor from the bottom of the generator column or a secondary fluid. Typically, a generator in a GAX aqua-ammonia absorption system will have a GAX desorber or an adiabatic desorber, but not both. When the system utilizes strong liquor GAX an adiabatic section is used, whereas a GAX desorber is used for weak liquor GAX or secondary fluid GAX. An additional component of a generator is a rectifier which is the section of the generator above the highest (coldest) feed point. Such generators as described above are illustrated in the drawings and will be described in further detail hereinafter.
The feed to the generator column from the system absorber is a rich liquor comprising a solution having a comparatively high ammonia content. Such rich liquor typically has 40% to 50% ammonia, but under some operating conditions may be as low as about 20%. Such a rich liquor is contrasted to a weak liquor directed from the generator to the absorber, which comprises a water-rich composition having between about 1% and about 15% ammonia at rating conditions, and typically between about 3% and about 5% ammonia. In conventional strong-liquor GAX absorption cycles, heat is recovered by passing a portion of the strong liquor through a heat exchanger in the GAX absorber, and heating the solution above its bubble point so that it becomes a two phase mixture. The portion of the strong liquor not passed through the GAX heat exchanger is introduced into the generator at or near the bottom of the rectifier as a single-phase liquid at or below the bubble point temperature. The second feed stream to the generator is that portion of the strong liquor which is passed through the GAX absorber. The second feed stream, comprising liquid and vapor, is introduced into the generator at a lower location than the first single-phase liquid feed. Conventional generators used in the aforesaid aqua-ammonia absorption systems have placed all the sections of the generator below the rectifier into a single shell, or multiple shells oriented one above the other such that they operated by gravity driving flow of liquid down the column from one section to the next. In some cases the rectifier is also positioned above the remainder of the generator (either in a separate shell or the same shell). In conventional systems wherein the rectifier was not placed completely higher than the remainder of the generator, a bubble pump, also know as a vapor-lift pump, has been used to pump liquid from the bottom of the rectifier to the top of the remainder of the generator.
The improved generator of the present invention is divided into a plurality of columns arranged so that the bottom of a lower temperature column is lower than the top of a higher temperature column. Eductors driven by feed streams are used to pump liquid from the bottom of one column to the top of the next column. In another embodiment of the invention, a separate rectifier section column is used with liquid from the rectifier directed to the inlet of the absorption system solution pump and pumped by the solution pump to the top of the stripping section column.